Recently, as mobile terminals prevail, become digitized, and have greater storage capacity, the demand for small and high-definition image sensors increases. In order to satisfy such a demand, it is indispensable to decrease the pixel pitch of image sensors. In the currently mainstream CMOS image sensor (hereinafter also referred to as “CMOS sensor”), the pixel size has been reduced to a 1.1 μm×1.1 μm low-pitch.
The pixel used in the aforementioned CMOS sensor is formed of a photodiode serving as a photoelectric conversion unit and a transistor serving as a readout circuit. The area occupied by the transistor serving as the readout circuit cannot be reduced below a certain ratio, even if the design rule is scaled down and the sharing of elements by adjacent pixels is advanced. At present, the ratio of the photoelectric conversion unit in a pixel (aperture rate) is said to be about 50%. That is, the reduction in pixel pitch would lead to the reduction of the area of the photodiode serving as a photoelectric conversion unit, thereby causing a problem of lowering the sensitivity of the sensor.
Recently, CMOS sensors outputting color images are widely used even in small mobile terminals. Generally, in a color CMOS sensor, one of the color filters transmitting any of red (R), green (G), and blue (B) wavelength bands is arranged on each pixel, so that a different pixel measures the light amount in each wavelength to obtain color information. Since a color filter absorbs light in wavelength bands other than the wavelength band of the color to be detected, about two-thirds of the entire light is discarded. In addition, generally, a color filter has a transmittance of about 80%. In consideration of the above, the degradation in sensitivity due to the color filters is also a problem.
In such a color filtering method, where only information for one of the colors R, G, and B can be obtained in one pixel, it is necessary to collect color information from adjacent pixels to perform a demosaicing process. For example, in a Bayer array, slight subject color variations close to the resolution of one pixel cause a defective mode called “false color” where incorrect color information is reproduced. Particularly, for the R and B filters for performing diagonal interpolation, it is only possible to obtain the spatial resolution √2 times the pixel pitch.
A known technique which focuses on the aforementioned point employs three stacked photoelectric conversion layers, with Ag dots with different pitch and width embedded in the respective photoelectric conversion layers. Sensitivity is improved and wavelength selectability is ensured using the plasmon resonant light between the incident light and the Ag dots.
However, several issues remain: the process of embedding metal dots in photoelectric conversion layers is complicated; defects taking place during the process of embedding cause an increase of dark current; and three-layer stack structure requires an improvement in sensitivity.